It is sometimes desirable to isolate a particular portion of one image and make a composite of that portion of the image with another image, such that the composite image appears to be a single image. One example of such a compositing technique uses a green screen. An actor is positioned in front of the green screen and a first image of the actor is captured. The green portions of the first image are then replaced by portions of a second image, making it appear that the actor was filmed in the environment of the second image. This technique has been used for, e.g., making a weather reporter appear to be standing in front of a wall-sized weather map.
The technique of using a colored background for image compositing is sometimes referred to as color-keying or chroma-key. The screen used in the chroma-key technique may be any color, and often is selected for its spectral distance from a subject of interest. A greater spectral distance allows more accurate isolation of the screen color from the image. For human subjects, blue and green screens are preferred because blue and green are considered to be far from human skin tone on the color spectrum. For non-human subjects, a color may be selected that is far on the color spectrum from the subject. For example, a green plant may be placed against a red screen to more accurately isolate the image of the plant from the red background.
It is not always desirable and/or practical to have a chroma-keyed screen available. It is therefore desirable to have an option for accomplishing an effect similar to the chroma-key technique effect without using a chroma-keyed screen.